866
JAMES [CONTRIBUTION FROM THE
J. LINGANE
Vol. 65
MALLINCKRODT CHEMICAL LABORATORY OF HARVISRD UNIVERSITY]
Pokrographic Characteristics of Stannous and Cupric Tartrate Complexes and Amperornetric Titration of Tin with Cupric Ion in Tartrate Medium BY JAMES J. LINGANE Recently, incidental to a study of amperometric titrations with titanous solutions, Strubl’ mentioned that the stannous tartrate complex ion produces an anodic wave in a saturated tartaric acid supporting electrolyte, but he did not supply further details. Since this anodic wave should have considerable value in connection with the polarographic determination of tin in the presence of other metallic elements with which it is commonly associated, the present investigation was undertaken to obtain more complete information about the polarographic characteristics of the stannous and stannic complexes present in various tartrate media. An amperometric titration of stannous tin with cupric ion in tartrate medium is also described. Experimental The usual polarographic technique was employed, a calibrated %gent-Heyrovsky Polarograph being used to record polarograms.’ The dropping mercury electrode was of the type described by Lingane and Laitinen,’ and the rate of flow of mercury was determined and checked a t frequent intervals by means of the volumetric instrument already described.‘ An H-type cell*~Swas used, with either a saturated calomel or mercury-mercurous d f a t e electrode as a working reference electrode. All measurements were performed with the cell in a water thermostat a t 25.00”, and air was removed from the solutions with purified nitrogen. Standard solutions of stannous chloride in 0.1 N hydrochloric or perchloric acid were freshly prepared as needed; they were standardized and checked a t each time of use by pipetting samples under a n excess of acidified ferric sulfate solution and titrating with standard ceric solution with o-phenanthroline ferrous complex as indicator. T o avoid air-oxidation of the stannous tartrate complex, the solutions investigated were made up in the cell, the stannous chloride solution being added last after air had been removed from the supporting electrolyte solution with nitrogen. The pH values of the tartrate solutions were determined with a glass electrode circuit.
Behavior of the Stannous Tartrate Complex.Polarograms of a relatively dilute solution of f 2 tin in 0.5 itl sodium potassium tartrate, with (1) R Strubl, Coll. Cscthoslov Chem Commun., 10, 490 (1938) (2) I M Kolthoff and J J Lingane, “Polarography,” Interscience Publishers, Inc , New York, N Y , 1941 ( 3 ) J J Lingane and H A Laitinen, I v d r : n p Chem , A n d E d , 11, 504 (1939) ‘4)J J Lingane, rbid , 14, 655 (1942)
and without gelatin present, are shown in Fig. 1. Cathodic current (reduction) is indicated by a positive sign and anodic current (oxidation) by a negative sign. It will be noted that the cathodic and anodic waves of curve a (without gelatin) both include prominent maxima, but very welldefined digusion currents are obtained following the maxima. Both maxima are completely eliminated by the addition of a small amount of gelatin (O.ooS%), as shown by curve b. Since there is no doubt that the cathodic wave corresponds to the reduction of the stannous tartrate complex to the metallic state, and since the cathodic and anodic diffusion currents are equal (see also Table II), it is evident that the anodic wave results from the oxidation of the stannous tartrate complex to the stannic state. The character of the polarograms is remarkably dzerent in acidic and alkaline tartrate supporting electrolytes, as shown in Fig. 2. From the curves in Fig. 2 , and the summary of half-wave potential data in Table I, it is seen that the cathodic and anodic half-wave potentials are both shifted to much more negative values in going from acidic to alkaline tartrate media. This marked PH effect indicates that there is a fundamental difference in the composition of the stannous complex in acidic and alkaline tartrate solutions. In acidic tartrate medium an U C ~ Qcomplex, i. e. containing coordinated hydrogen tartrate ion or tartaric acid molecules, probably predominates.
TABLE I HALF-WAVEPOTBNTIhLS OF STANNOUS TIN IN TARTRATE MEDIUMAS A FUNCTION OF pH The concentration of stannous tartrate complex was 6.45 X 1 0 - 3 .M iri all cases, and O . O l ~ ogelatin was present as a maximum suppressor. The values of the half-wave pate;tials are referred to the saturated calomel electrode a t 25 . Supporting electrolyte. molar HrCc- NaHC4- N a G H.06 H,Oe HcOr Other salts 0 40 0.10 . 3 8 0 . 0 4 7 0 . 3 8 NaCl .lI .36 . l l NaCIO4 43 .IO NaClOa .0017 NaOH 43 .lONaOH .47 .2NaOH .48
pH 2.3‘ 3.44 4.3‘ 9.0’ 9.8’ 13 Oc 13 3O
volts an. ca. - 0 . 1 4 -0.49 .20 .54 .28 .59 .33 92 .34 .95 .71 1 16 .77 1 18
Measured with glass electrode. * From transition color of thymol blue. Calculated from concentration of sodium hydroxide. (I
AMPEROMETRIC TITRATION OF TINWITH CUPRICIONIN TARTRATE
May, 1943 b;
867
ai I
1
3
10;
20-
6;
10-
0.
b
r
.
-5; -10
i
PH values mixtures of various species appear to be
+40-
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JAMES
J. LINGANE
Vol. 65
plex is thermodynamically irreversible at the dropping electrode. The irreversibility was confirmed by investigating polarograms of sfnnnic tin in tartrate media. In neutral or alkaline tartrate solutions stannic tin does not show a reduction wave below the potential at which sodium ion is discharged, whereas if the stannousstannic system were reversible a reduction wave would be observed with stannic tartrate solutions at the same potential a t which the anodic wave occurs with stannous tartrate solutions. Judging from the slope of the cathodic w a w , the reduction of the stannous tartrate complex is also irreversible at the dropping electrode, but much less so than the oxidation process.
the anodic diffusion current is excellently defined and quite normal. At a pH of about S-9, corresponding approximately to the pH of a pure solution of disodium tartrate (curve b), the anodic diffusion current is somewhat smaller than in the strongly alkaline medium; this is easily explicable by a change in the nature of the tartrate complex (wide supra) with a corresponding change in diffusion coefficient. However, it will be noted that the anodic diffusion current in curve b decreases abruptly prior to the anodic dissolution current of the mercury, so that a pronounced minimum occurs in the curve. This minimum is even more evident in a tartaric acid-sodium hydrogen tartrate supporting electrolyte (curve d in Fig. 2 ) . The minimum is only slightly noticeable in curve f hwvlvMw c of Fig. 2; probably be'4vlvMww cause of the relatively large I b concentration of chloride , I ion (0.38 M ) present in this I solution, which depolarizes I the dropping electrode a t d C a potential more negative r n than that a t which the minimum is fully developed. Similar, although less prominent, minima have been observed in the anodic waves of hydroquinone5and of reduced rhenium solutions6 Hence the present +0.4 ~ 0 . 1 -0.2 -0.5 -0.8 -1.1 case is not unique, and the +0.4(4 +0.4(c) occurrence of such a miniEd.e.vs. s. c. E., volts. Fig.3.-Various concentrations of stannous tin in 0.38 M disodium tartrate, 0.12 M mum does not appear to be sodium hydrogen tartrate, and 0.01% gelatin. Concentrations of tin were (a) 1.88, (b) simply related to the type 3.68, (c) 7.08, and (d) 14.6 millimolar. Galvanometer sensitivity was 0.670 microamp. of that is oxidized per mm. for curves a, b and c; 1.675 microamp. per mm. for curve d. at the dropping electrode. In acidic tartrate solutions the anodic wave is Since the decrease in current causing the minimum characterized by a remarkably steep slope; by occurs near the potential a t which oxidation of manual manipulation of the polarograph it was the mercury of the dropping electrode begins, found that a change of only 5 or 6 millivolts in apparently the oxidation of the mercury inthe vicinity of the half-wave potential caused hibits the oxidation of other substances. It is sigthe anodic current to drop from its limiting value nificant that the minimum is much less prominent to practically zero. This indicates that what- in the presence of a high concentration of chloride ever the slow step in the oxidation process may be ion, and completely absent in the presence of it requires a very sharply limited activation hydroxyl ion (curves c and a in Fig. 2), in which energy. cases the dropping electrode is depolarized a t a It is seen from the curves in Fig. 2 that the relatively positive potential and the products of character of the anodic diffusion current differs the oxidation of the mercury are mercurous chlo0. H Muller and J P. Baumberger, 7'ranr 4111 Eleclrorhr,a markedly in acid and alkaline tartrate solutions. b (5) c , 71, 169, 181 (1937) In alkaline tartrate medium (curve a in Fig L ' i i t ) J J Tinganr Tirir n'. 64, 2182 (I'Jld) I
